1. Field of the Invention
This invention relates to a method and apparatus for controlling motors. More particularly, the invention relates to a method and apparatus for controlling motors suitable for application to image formation, such an image-exposure drive motor, a polygon mirror drive motor and a photosensitive body drive motor.
2. Description of the Related Art
Conventionally, velocity control in an apparatus of this type is carried out by a phase-locked loop (referred to as a PLL). In such case, an internal constant (gain) of PLL control is changed over between a value which prevails in control of a transitional period, such as when a motor is started, and stopped, and has its direction of rotation changed, and a value which prevails in control under steady conditions following attainment of a target rotational speed. By adopting such an expedient, the supply of current to the motor at a transitional time is limited to suppress overshoot and effect a smooth transition to the steady state.
However, since the supply current to the motor at the transitional time is limited, there is an increase in the length of the transitional period. As a consequence, in the case of an image-exposure drive motor, the approach distance from the start to the leading edge of an image must be set long when the motor is started This leads to an apparatus of a large size. Another problem is that the length of the transitional period has an effect upon the period from exposure of one image to exposure of the next image In the case of a polygon mirror drive motor or photosensitive body drive motor, the length of the transitional period to attain a target rotational speed is increased at motor start-up or when changing over a target velocity. As a result, there is an increase in fast-copy time or in the time needed for reproduction requiring a changeover in speed.